The spleen of the ex-hypoxic polycythemic mouse will be used as a model to study the effects of erythropoeitin and androgens (testosterone and etiocholanolone) on erythropoietin-responsive cells. The effect of erythropoietin on histone acetylation and deacetylation will be examined in relationship to DNA synthesis and previously described changes in RNA polymerase activity and histone synthesis. Early changes in protein synthesis, following administration of erythropoietin, will also be investigated with attention to the possibility of selective synthesis of a particular protein or class of proteins. The interaction of androgens with the nuclei of erythroid cells will be studied with reference to the requirement for a cytoplasmic receptor protein and the ability of different androgens to stimulate nuclear RNA polymerase activity in erythroid cells. The effect of androgens on hemoglobin switching in sheep marrow cells in culture will also be investigated.